1. Field of the Invention
The present invention relates to a lens, mirror, prism, or similar plastic molding produced by injection molding and included in an optical device, e.g., a copier, laser printer, facsimile apparatus or similar image forming apparatus, and a method and apparatus for producing the same. More particularly, the present invention is concerned with a plastic molding having, e.g., mirror surfaces and a fine undulation pattern transferred thereto with high accuracy by injection molding, and a method and apparatus for producing the same.
2. Discussion of Background
For injection molding, it is a common practice to use a mold assembly including a mold surface forming a cavity having a preselected volume, a transfer surface formed on the mold surface for transferring a mirror surface to a molding, and a gate open at the mold surface and having a preselected opening area. Molten resin is injected into the cavity via the gate and then cooled. The resulting molding is taken out by opening the mold assembly. While such a molding, particularly a mirror, lens, prism or similar optical element, is required to have an accurate mirror surface and a uniform refractive index, the mirror surface needing a high surface accuracy is caused to sink because the molten resin contracts at the time of solidification.
Injecting molding methods for solving the above-described problem are taught in, e.g., Japanese Patent Laid-Open Publication Nos. 3-128218, 8-234005, 3-151218, and 3-281213 (Prior Art 1 hereinafter). In prior art 1, a non-transfer surface or mold surface, which faces a transfer surface, is formed with, e.g., a roughened mirror surface or a surface treatment for lowering wettability. Otherwise, use may be made of a porous material. Injection is stopped just before a cavity is filled up with molten resin. Then, the molten metal is solidified by cooling without any dwelling. As a result, the roughened surface is caused to sink due to a difference in adhering force between the molten resin, the transfer surface, and the roughened surface. This prevents the mirror from sinking. Alternatively, an overflow portion for receiving excess molten resin is located outside of the cavity. When the overflow portion begins to be filled, injection is stopped. Then, the molten resin is solidified by cooling without any dwelling. This also allows the roughened surface to sink due to a difference in adhering force between the resin, the transfer surface, and the roughened surface.
An injection molding method disclosed in Japanese Patent Laid-Open Publication No. 2-175115 (Prior Art 2 hereinafter) injects molten metal into a cavity in which a porous member, which communicates with a compressed gas, is provided on a mold surface contacting the nontransfer surface of a molding. While dwelling and cooling are under way after the injection of the molten resin, air is fed to the non-transfer surface of the molding via the porous member. With this method, a side of a cylindrical thin lens may be caused to sink.
Japanese Patent Laid-Open Publication No. 6-304973 (Prior Art 3 hereinafter) proposes an injection molding method in which a non-transfer surface communicates with the outside air via a vent hold. During an interval between the beginning and the end of the injection of molten resin into a cavity, a pressure difference is generated between the transfer surface and the nontransfer surface of the resin. As a result, the non-transfer surface of the resin is caused to sink. Specifically, air is brought into contact with the molten resin, on a surface other than the mirror portion transferred from the transfer surface, via the vent hole and a bore communicating therewith, so that the cooling speed of the resin is lowered. At the same time, a preselected air pressure is fed to the vent hole in order to generate a preselected pressure difference between the mirror portion of the resin and the vent hole. This allows only the portion of the resin facing the vent hole to sink, i.e., prevents the mirror portion form sinking. In addition, because only the vent hole portion of the resin sinks, a molding can be produced by simple control over the amount of the resin to be injected into the cavity and without any strain being generated in the resin. The resulting molding is therefore free from an internal strain and is provided with an accurate mirror surface.
Prior Art 3 further teaches that the vent hole may communicate with a compressor so as to apply a preselected air pressure to the vent hole portion of the resin. With this configuration, it is possible to generate any desired pressure difference between the mirror surface portion and the vent hole portion of the resin, thereby causing the vent hole portion to sink. In addition, the pressure difference is readily adjustable in order to further enhance the accuracy of the mirror surface without any internal strain.
Japanese Patent Laid-Open Publication No. 6-315961 (Prior Art 4 hereinafter) teaches an injection molding method causing the non-transfer surface of resin to sink. In accordance with this method, the transfer surface of a mold is heated to and held at a high temperature. The transfer surface side of the resin is heated to a high temperature until the injection of molten resin into a cavity ends.
However, Prior Art 1 relying on the roughened surface, surface treatment or porous material results in an expensive mold assembly. Moreover, stopping the injection just before the cavity is filled up with the molten resin is extremely difficult. Should the correct timing for stopping the injection of molten resin not be realized, the relationship in the adhering forces between the transfer surface and the roughened surface would be inverted and would thereby cause the mirror surface to sink or result in being short of resin. In addition, because sinking cannot be provided with directionality and because setting the molding conditions is difficult, the configuration of the molding is critically limited. It is more preferable that the injection of the molten resin be stoppable at any time lying in a broader range. However, in this case, the overflow portion, which is formed integrally with the molding, must be removed by an extra step. Moreover, if the area of the opening of the gate for feeding the molten resin to the overflow portion be excessively small, the relation in adhering forces between the transfer surface and the roughened surface would also be inverted and would thereby cause the mirror surface to sink and there would not be enough molten resin.
Prior art 1 can implement a mirror or similar optical element needing a single mirror surface because it roughens the mold surface facing the transfer surface. However, Prior Art 1 cannot produce a lens, prism or similar optical element because the number and the positions of the mirror surfaces are limited. In addition, the relation in adhering force is inverted and causes the mirror surface to sink, depending on the material constituting the transfer surfaces and roughened surface and the kind of resin used.
Prior Art 2 increases the cost of the mold assembly due to the porous member and provides more sophisticated control over the configuration of the porous member. Specifically, if the effect of the porous member is excessive, it not only admits the molten material thereinto, by also obstructs the parting of the molding. This is particularly true when the porous portion of the porous member extends inwardly over the wall of the mold. Further, because the compressed gas is fed to the non-transfer surface of the molding via the porous member during the previously stated interval, a pressure difference is maintained between the non-transfer surface and the transfer surface of the resin during cooling. As a result, the internal strain remains in the resulting molding after the opening of the mold. The residual pressure not only lowers the accuracy of the transfer surface, but also causes the entire molding to deform.
Prior Art 3 generates a pressure difference between the transfer surface and the nontransfer surface of the resin during the interval mentioned earlier. This also brings about the problem stated above in relation to Prior Art 2. Prior Art 4 maintains the transfer surface of the mold at high temperature and heats the transfer surface side of the resin to a high temperature during the previously mentioned interval. This is also undesirable in the above respect.